1. Field of the Invention
This invention relates to the construction of fuel cell stacks, and more particularly to the construction of fuel cells, such as oxygen-ion conducting solid oxide fuel cells and proton conducting ceramic or polymer membrane fuel cells, in which the electrolyte is a solid.
2. Description of the Prior Art
Fuel cells with solid electrolytes are the most promising technologies for power generation. Solid electrolytes are either ion-conducting ceramic or polymer membranes. Ceramic oxygen-ion conducting membranes are primarily based on zirconia but other electrolytes for lower operating temperatures are under development. The most advanced construction with ceramic membranes is the tubular solid oxide fuel cell based on zirconia. The tubular construction can be assembled into large units without seals and this is its biggest engineering advantage. Tubular solid oxide fuel cells are fabricated by electrochemical vapor deposition processes, which are slow and costly. The tubular geometry of these fuel cells limits the specific power density, both on weight and volume basis, to low values while the electron conduction paths are long and lead to high energy losses due to internal resistance heating. For these reasons, other constructions are actively being pursued at the present time.
The most common alternative construction to the tubular construction is the planar construction which resembles a cross-flow heat exchanger. The planar cross flow fuel cell is built from alternating flat single cell membranes, which are trilayer anode/electrolyte/cathode structures, and bipolar plates, which conduct current from cell to cell and provide channels for gas flow, into a cubic structure, or stack, which is manifolded externally on four faces for fuel and oxidant gas management. At the very minimum, the single cell membrane and the bipolar plate of every cell in the stack must be sealed together gas-tight at each manifold face and the manifolds must be sealed gas-tight to the stack to prevent fuel and oxidant gas cross leakage. Fuel and oxidant gas cross leakage compromises fuel cell efficiency and is hazardous due to the mixture's potential for explosion. Sealant materials with thermal expansion coefficient matching those of the other components of the stack and with satisfactory lifetime at the operating temperatures of about 1000.degree. C. are not available at the present time, and this represents a serious technological shortcoming for planar solid oxide fuel cells. Planar fuel cell stacks may, at least in theory be constructed in a coflow configuration, but no realistic construction exists for external manifolding because of the extremely short dimensions of the gas channels required for high power densities.
U.S. Pat. No. 4,490,445 teaches a solid oxide fuel cell construction in which the single cells and the conductor, or interconnector, plates have a circular footprint. In this construction the conductor plates have groove networks on both major faces which are formed by mostly circular ridges. The grooves define gas passages for the flow of gases. For example, fuel gas is introduced at the periphery of the conductor plate into the space defined by the conductor groove network and the anode side of the single cell, flows into the groove network through a line of notches in the ridges, then to an exit opening, located at the diametrically opposite end of the plate, and from there the partially reacted fuel gas stream is channeled to the next conductor/anode gas space. The overall flow pattern of the fuel gas throught the groove network is mixed radial/circumferential pattern but is mostly circumferential. The ridges of the conductor plates contact the single cell electrode faces for current conduction. Each of the conductor major faces is coated with a coating having the same composition as the electrode being in contact with. Moreover, these conductor plates have circumferential ridges arranged along the edges of said conductor plate to define gas seals. In this construction, there are fuel and oxidant streams in and out of the stack, these streams are in counterflow, and, on the fuel side, each successive cell is contacted by progressively more reacted fuel stream. In summary, U.S. Pat. No. 4,490,445 refers to a very specific SOFC construction, i.e., a construction with conductor plates having groove networks formed by ridges to define gas passages wherein these conductor plates have circumferential ridges arranged along the edges of the conductor plate to define gas seals and the primary surfaces of these conductors have surface coatings of the same composition as the electrodes they are in contact with.
U.S. Pat. No. 4,629,537 is a sequel to U.S. Pat. No. 4,490,445 and teaches fabrication of the solid oxide electrolyte plate by plasma spraying, the plate thickness ranging from about 50 to 750 micrometers, and assembling fuel cell stacks by stacking alternating layers of electrolyte and interconnector plates, as defined in U.S. Pat. No. 4,490,445, together.
U.S. Pat. No. 4,770,955 teaches a solid oxide fuel cell construction with gas-impervious electrolyte and separator plates and powder (or partially-sintered), i.e., porous, anode and cathode layers which make contact with the electrolyte and the separator plates, and gas-impervious tubular gaskets for gas sealing. In this construction the anode and cathode layers act as the electrodes and the cell-to-cell interconnects at the same time. The gas-impervious tubular gaskets, when properly bonded to the electrolyte and separator plates around openings corresponding to the internal diameter of the tubular gaskets, define axial conduits for fuel and oxidant gas distribution to the anode and cathode layers, respectively. Moreover, this construction requires semi-impervious sealing at the periphery of he anode layer. One of the inherent weaknesses of this construction is the lack of good flow control. Gas flow is controlled by the electrode layers, i.e., by the porosity of the powder or partially-sintered electrode layers and the length of these layers in the direction of flow. The unsymmetrical location of gas inlet conduits, gives rise to differing flow path lengths and, thereby, non-uniform flow distribution over the cathode and anode electrodes.
U.S. Pat. No. 5,399,442 teaches a solid oxide fuel cell construction with metallic interconnector plates, dense ceramic disks separating the interconnector plates, and an annular single cell trilayers. The interconnector plates, have a manifold part and a reaction part which is made up of guide vanes on either fiat surface of the plate. These guide vanes create a sequence of dams, so to speak, which are notched to allow for the passage of the reaction gases from one dam to the next and ultimately out at the interconnector circumference. A dense ceramic disk provides electrical insulation between the metallic interconnectors. The ceramic disks and the single cells are sealed to the interconnector by seal materials which are mixtures of glass and ceramic which liquefy at the fuel cell operating temperature of about 1000.degree. C. The metallic interconnectors are provided with two drilled holes to allow passage of the inlet gases from feed channels over the single cell electrodes.
Proton ion-conducting membranes or proton electrolyte membranes (PEM) are primarily based on Nafion.RTM. but other materials are under development for slightly higher operating temperatures. Fuel cells based on PEM are put together in a planar configuration very much like the planar construction described above for the solid oxide fuel cells. Due to the much lower operating temperature, i.e., about 80.degree. C., of PEM fuel cells sealing is not a major problem, but the technology would benefit from a construction that is easy to assemble and manifold.